Carbon Sequestration Using a Floating Vessel

ABSTRACT

Disclosed is a method for removing carbon dioxide from the atmosphere. The method comprises the step of delivering urea from a floating vessel to a region of a photic zone of the ocean, whereby the number of phytoplankton is caused to increase in the region upon addition of the urea. The method can be used for producing carbon cells.

TECHNICAL FIELD

Disclosed is a method for removing carbon dioxide from the atmosphere.

BACKGROUND ART

The carbon dioxide concentration in the atmosphere is rising as a resultof fossil fuel burning, bringing with it the threat of undesirableclimate change. At the same time, the fishing industry seems to beexploiting the ocean's resources beyond its carrying capacity. Increasedphotosynthesis may address both of these issues by converting inorganiccarbon (carbon dioxide) to organic carbon (vegetable matter) in a bodyof water. Such vegetable matter is the base of the marine food chain.

The natural process by which carbon dioxide is converted into organiccarbon is known. When atmospheric carbon dioxide dissolves in the oceanit exists in an ionic form and can be taken into the bodies of marinephytoplankton through the process of photosynthesis. The phytoplanktoneventually perish through age or are eaten by other marine organisms.Some of the resulting dead or excreted biomass then falls to the lowerlevels of the water column whereby at least some of the carbon iseffectively stored away from the atmosphere for long periods of time.

In some regions of the ocean, the conversion of carbon dioxide dissolvedat the surface of the ocean to organic carbon is limited by theavailability of specific nutrients. For example, phytoplankton growth islimited in some 80% of the ocean by the lack of the macronutrientnitrogen.

SUMMARY OF THE DISCLOSURE

In a first aspect, there is provided a method for removing carbondioxide from the atmosphere. The method comprises the step of deliveringurea from a floating vessel to a region of a photic zone of an ocean,whereby the number of phytoplankton is caused to increase in the regionupon addition of the urea.

This method involves delivering an exogenous source of nitrogen (i.e.urea) to a specific layer of the ocean at a specific location tostimulate the growth of the phytoplankton population in that region andso cause an increase in the photosynthetic activity of the phytoplanktonpopulation. This method can be utilised to decrease the atmospheric CO₂concentration because, as inorganic carbon present in the photic zone ofthe ocean is used in the photosynthesis carried out by thephytoplankton, further CO₂ is caused to diffuse into the ocean from theatmosphere. The increased number of phytoplankton will eventually dieand fall to the lower levels of the water column. Accordingly, theaddition of urea to the photic zone can cause the ocean to be a sink forCO₂ and can result in a reduction in the CO₂ concentration in theatmosphere.

As the skilled addressee will appreciate, since phytoplankton is thebase of the marine food web, increasing numbers of phytoplankton mayalso result in an increase in ocean fish biomass.

In the method described above, urea is delivered to the photic zone ofan ocean from a floating vessel. Delivering a source of nitrogen (i.e.urea) from a floating vessel has a number of advantages over othermethods of delivery, such as via a pipeline. For example, it might notbe economically viable to provide the nutrient by pipeline because ofthe extensive length of pipe required to reach an appropriate region ofthe ocean, as well as the ongoing costs of maintaining the pipeline.Furthermore, a land based facility is required for pumping the source ofnitrogen through the pipeline, which would further increase the cost.

Furthermore, as noted earlier, phytoplankton growth is limited in some80% of the ocean by the lack of the macronutrient nitrogen. A portablefloating vessel can assist in the delivery of a source of nitrogen (i.e.urea) to such regions.

Urea can be supplied as a granular white material and is extensivelyused in agriculture. Urea occurs naturally in sea water as a result ofthe bacterial decay of dead phytoplankton or zooplankton excretions. Ithas numerous advantages over many other nitrogen containing compounds,such as ammonia, in that it can be easily stored and transported, is notcaustic and is pH neutral. In contrast, ammonia (or solutions ofammonia) is caustic, toxic and classified as a dangerous chemical.

Typically, the region is located such that carbon dioxide is sequesteredfrom the atmosphere for a significant period of time (e.g. greater than100 years). The deeper the dead phytoplankton and other derived organicmatter can sink out of the photic zone, the longer the carbon is keptaway from the surface area of the ocean. Thus, in at least preferredembodiments, the urea is delivered to the photic zone in a region of theocean having a depth sufficient to allow dead said phytoplankton andorganic material derived from the phytoplankton to fall from the mixedlayer and enable carbon from the CO₂ to be sequestered from theatmosphere for considerable time (or alternatively, to a location whereocean currents will carry the source of nitrogen and additionalphytoplankton to such a region).

The region may, for example, be located at the edge of a continentalshelf or over a deep ocean, where the organic matter can sink to a depthof 1000 m or more.

In some embodiments, the urea is transported on the floating vessel ingranular form and is mixed with water taken from the region immediatelybefore delivery. As a person skilled in the art will appreciate, theproperties (e.g. temperature, density and salinity) of a solution ofurea dissolved in such seawater will be similar to those of the seawaterinto which the urea is to be delivered. Indeed, if the solution of ureais a relatively dilute solution, its properties will be very similar tothat of the surrounding seawater. As such, the seawater containing theurea will not appreciably rise or fall in the water column before thephytoplankton are able to access the nitrogen in the urea.

The urea is typically injected into the region at a predetermined depth(e.g. between 15 and 50 m) in order to form a concentrated solution atthe depth most suitable for phytoplankton growth. This depth can bedependent on a number of factors, for example, few phytoplankton existbelow 100 m because very little sunlight penetrates that deep into theocean. Similarly, few phytoplankton exist in the top few meters of theocean because the sunlight is too intense.

In alternate embodiments, the urea is in granular form and is sprinkledfrom the floating vessel over the surface of the ocean above the region.The granules of urea may contain air pockets inside the granules, whichslows the rate at which the granules sink so the nitrogen does not sinkout of the photic zone.

In some embodiments, the amount of urea delivered to the region resultsin the concentration of urea in the region being between about 0.1μmole/L to about 10 μmole/L. Concentrations of less than about 0.1μmole/L urea do not cause a significant growth in the number ofphytoplankton, while concentrations above about 10 μmole/L urea cancause too much phytoplankton growth. To achieve such a concentration ofurea, a 5% w/v to 20% w/v solution of urea in seawater can be pumpedinto the photic zone to raise the concentration of urea in the immediatevicinity of the outlet. This concentrated urea solution willsubsequently be dispersed by ocean currents and turbulence to provide aconcentration of urea sufficient to support an appropriate amount ofphytoplankton growth.

Alternatively, the concentration of urea may be raised by between about0.1 μmole/L to about 1 μmole/L, or about 1 μmole/L to about 10 μmole/L,or about 0.1 μmole/L to about 0.5 μmole/L, or about 0.5 μmole/L to about1 μmole/L, or about 1 μmole/L to about 5 μmole/L, or about 5 μmole/L toabout 10 μmole/L, of seawater.

In some embodiments, one or more additional macronutrients (e.g.phosphates) may be delivered to the photic zone with the urea.

In some embodiments, one or more micronutrients (e.g. iron) may bedelivered to the photic zone with the urea.

In some embodiments, the method of the first aspect comprises theadditional step of monitoring the increased number of phytoplankton, andadding more urea to further increase or maintain an increased number ofphytoplankton.

In a second aspect, there is provided a method for removing carbondioxide from the atmosphere. The method comprises the steps ofdelivering a source of nitrogen from a floating vessel to a region ofthe photic zone of an ocean, whereby the number of phytoplankton iscaused to increase in the region upon addition of the source ofnitrogen, monitoring the increased number of phytoplankton, and addingmore of the source of nitrogen to portions of the region in which it ispossible to further increase or maintain an increased number ofphytoplankton.

The increased number of phytoplankton may, for example, be monitored bysatellite. Alternatively, the increased number of phytoplankton may bemonitored by a second boat or ship downstream of the floating vessel.

In some embodiments, a dye such as hydrogen hexafluoride may bedelivered to the photic zone with the source of nitrogen to aid inmonitoring the increased number of phytoplankton.

Preferably, the source of nitrogen used in the method of the secondaspect is urea. However, in some embodiments, notwithstanding thedifficulties described above, the source of nitrogen used can be ammoniaor one of its salts (either in solution or in the gas phase). Ammoniaalso occurs naturally in sea water as a result of the bacterial decay ofdead phytoplankton or zooplankton excretions. Other sources of nitrogensuch as sodium nitrate and nitric acid may also be used in the method ofthe second aspect.

In a third aspect, there is provided a method for producing carboncredits comprising the step of removing carbon dioxide from theatmosphere using the method of the first or second aspect.

The use of such a method for producing carbon credits may make it highlydesirable to industries which require their carbon emissions to beoffset.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Specific embodiments of the methods set forth in the summary will now bedescribed.

In the methods described above, carbon dioxide is sequestered from theatmosphere by adding a source of nitrogen (e.g. urea) to a region of thephotic zone of an ocean to cause an increase in the number ofphytoplankton. Such activity is also known as Ocean Nourishment (whichis a trade mark of Earth, Ocean and Space Pty Ltd), which is thepurposeful introduction of nutrients to the upper ocean for the purposeof storing carbon and enhancing the sustainable supply of marineprotein.

Typically, the photic zone (also known loosely as the mixed layer)extends from the surface of the ocean to a depth of about 50 metres.However, the photic zone may extend to a depth of 100 metres or more.The actual depth of the photic zone varies, and is dependent upon anumber of factors including wind strength and the loss of heat due tothe temperature difference between the oceanic surface waters and thelower atmosphere.

The region referred to above must be a region of the ocean which isdeficient in nitrogen in order for the addition of the source ofnitrogen to cause the number of phytoplankton to increase. As notedabove, it is thought that phytoplankton growth is limited in up to 80%of the ocean by the lack of the macronutrient nitrogen. It is within theability of the person skilled in the art to determine appropriateregions to fertilise in accordance with these methods.

When using these methods, the organic carbon produced by photosynthesiscan be exported from the photic zone to the deep ocean and retainedthere for some time. This occurs either by subduction of surface watersor by sinking of particulate organic detritus out the upper ocean. It isbelieved that carbon dioxide may be sequestered from the atmosphere forat least 100 years, and possibly for up to 500 years or 1000 years, oreven longer, depending on the location and ocean currents.

In the first aspect, and in some embodiments of the second aspect, ureais delivered to the region from the floating vessel. Urea is highlysoluble and, when involved in photosynthesis in the ocean, can cause theupper ocean to become less acidic. Adding urea to the ocean maytherefore counter the effect of increased levels of carbon dioxide inthe atmosphere causing the pH of the upper ocean to decrease.

Typically, the amount of the nitrogen containing compound (e.g. urea)delivered to the region results in the concentration of the nitrogencontaining compound in the region being between about 0.1 μmole/L toabout 10 μmole/L. To achieve such a concentration, a concentratedsolution (e.g. a solution containing 5% w/v of urea) of the nitrogencontaining compound in seawater can be pumped into the photic zone toraise the concentration of the compound in the immediate vicinity of theoutlet. This concentrated patch is subsequently dispersed by oceancurrents and turbulence to provide an appropriate concentration ofavailable nitrogen to support phytoplankton growth.

The concentration of the nitrogen containing compound may be raised bybetween about 0.1 μmole/L to about 1 μmole/L, or about 1 μmole/L toabout 10 μmole/L, or about 0.1 μmole/L to about 0.5 μmole/L, or about0.5 μmole/L to about 1 μmole/L, or about 1 μmole/L to about 5 μmole/L,or about 5 μmole/L to about 10 μmole/L, of seawater.

An example of how specific embodiments of the methods set forth abovecould be carried out will now be described.

The hold of a floating vessel in the form of a ship could be loaded withurea, preferably in the form of granular urea, which is not classifiedas a dangerous chemical and has suitable storage stability and flowproperties. The ship could then sail to any region of the ocean in whichthe phytoplankton population would be caused to increase upon additionof urea and where the organic carbon produced would sink to a depthwhere it would not be quickly recirculated back to the surface. Forexample, the ship could be anchored at the edge of a continental shelfor floating over the deep ocean.

Once the ship was located at the desired region of the ocean, the ureacould be blown into a venturi, mixed with seawater to provide a solutionof about 5% w/v urea, and the resultant solution injected via a pipefrom the ship to a depth of the photic zone that is determined to bemost suitable for phytoplankton growth in that region.

The seawater used to dissolve or suspend the urea would ideally be takenfrom the region of the photic zone to which the urea was to be injectedso the injected water will have a similar temperature and density to thesurrounding water. If the injected solution/surrounding water havesimilar properties, then the urea will remain in the photic zone atapproximately the level at which it was injected for long enough to bediluted by natural processes, thus enabling the phytoplankton tophotosynthesise. Should the urea sink out of the photic zone it would nolonger be accessible to the phytoplankton. Similarly, should theinjected solution be significantly less dense than the surroundingseawater, it could float towards the surface where the urea may alsobecome inaccessible to phytoplankton because phytoplankton do not tendto exist close to the surface of the ocean where the sunlight is toointense.

The predetermined depth at which the urea/seawater solution was injectedwould depend on the depth of the photic zone in the region, which couldreadily be calculated on the ship before injection, but which wouldtypically be between about 15 and about 50 metres.

Alternatively, a dense solution of urea and seawater could be sprayedout into the water, where it would subsequently sink to near the centreof the photic zone. As noted above, phytoplankton tend to live near themid-depth of the photic zone.

Alternatively, an appropriate quantity of the urea in granular formcould be sprinkled from the floating vessel over the surface of theocean, where it would subsequently sink to near the centre of the photiczone whilst dissolving.

In some embodiments, the urea may be provided in the form of small,spherical pills having an air bubble inside. The air bubble would slowthe rate of sinking to allow for complete dissolving of the urea whilethe pill is in the photic zone. The rate of sinking of these pills(which depends on pill density and diameter) could be estimated usingStokes law.

Alternatively, the urea could be mixed with fresh water at the dock inorder to achieve a solution having a satisfactory density.

The ship could deliver the urea as it steams along a predetermined path,chosen to produce an extensive phytoplankton patch in a desirablelocation. Alternatively, the ship could steam in a grid patternthroughout the region.

Additional macro- or micronutrients (e.g. phosphorous or iron) can beadded to the urea solution at an appropriate concentration if needed.Alternatively, such macro- or micronutrients could be delivered to theocean separately.

Approximately a week after the source of nitrogen has been deliveredinto the ocean, a phytoplankton patch will occur. The existence of thispatch can be monitored down current of the injection points, for exampleby satellite or by a second boat (or by the ship which originallyinjected the urea) and the concentration of the phytoplankton can bemeasured.

If desirable, the ship (or another ship) containing urea could return tothat region of the ocean and add additional amounts of urea to furtherincrease or maintain the increased number of phytoplankton in thedesired region.

The released urea forms a nutrient plume in the photic zone, which isspread by the ocean mixing throughout the photic zone. Ocean currentsand diffusion assist in the dispersion of nitrogen through the region.The presence of the added nitrogen together with sunlight enables thephytoplankton in the photic zone to multiply as the source of nitrogen(e.g. urea) and other added nutrients or naturally occurring nutrientsare consumed. In this manner, phytoplankton patches of substantial sizecould be maintained for some time.

Dead phytoplankton and organic material comprising excretion fromzooplankton subsequently fall to lower levels of the water column (i.e.the deep ocean) as organic detritus, and the ocean current carries theurea and phytoplankton over the sea bed. The organic detritus carrieswith it carbon originating from carbon dioxide from the atmosphere,enabling effective sequestering of carbon to deeper ocean layers or thesea bed. As the amount of carbon dioxide that could be sequestered fromthe atmosphere is proportional to the amount of additional phytoplanktonproduced, phytoplankton patches of substantial size and which aremaintained for some time could sequester large amounts of carbondioxide.

It will be appreciated that delivering a source of nitrogen (e.g. urea)from a floating vessel offers numerous advantages over the use of fixedpipelines to deliver the source of nitrogen. For example, as describedabove, a floating vessel could be used to cause a long-lastingphytoplankton patch over a large region of the ocean despitefluctuations in the currents advecting the urea from the point ofinjection.

Whilst urea is a preferred source of nitrogen, other compounds which aresources of nitrogen (e.g. ammonia) could be used in the method of thesecond aspect set out above. As noted above, compounds such as ammoniaare less preferred because they are more difficult to handle than ureaand are, in fact, classified as dangerous chemicals. However, in someembodiments of the second method, such problems could be overcome andammonia could be added to the photic layer by bubbling gaseous ammoniafrom an outlet located beneath the ship. Alternatively, ammonia insolution could be sprinkled either onto the surface of the ocean, whereit sinks into the photic zone, or directly into the photic zone at thepredetermined depth from the ship.

Furthermore, additional helpful macronutrients (e.g. phosphates) ormicronutrients (e.g. iron) could also be added to the urea/other sourceof nitrogen and delivered to the photic zone if it were determined thatthe presence of such nutrients may further increase the number ofphytoplankton.

A specific embodiment of the method described above that was performedin the South Pacific Ocean will now be described.

A ship carrying urea in 40 kg bags sailed to a region in the high seasto confirm the utility of the methods described above. The urea (10 kg)was dissolved in a 60 litre container of seawater taken from the region.Phosphorus, in the form of superphosphate, was also dissolved in thecontainer in the Redfield ratio. The density of the resultant solutionwas 1075 kg/m³. The solution was then poured onto the sea from the ship.This was repeated three times in different locations.

To verify that a solution containing urea and superphosphate waseffective in promoting a growth in the phytoplankton population ofseawater taken from the locations (i.e. dissolved inorganic carbon isconverted into organic carbon), two samples of the ambient sea waterwere collected from each location. The initial chlorophyllconcentrations in the locations was 0.25 μg/L, 0.25 μg/L and 0.1 μg/L.As those skilled in the art will appreciate, these are relatively lowconcentrations of chlorophyll in seawater, and these regions can bethought of as “desert” regions of the ocean.

One sample from each location was dosed with urea and superphosphate inthe Redfield ratio, and the other was maintained as a control. Thesamples were exposed to natural sunlight and kept in a water bath for 4days. At the end of the period, the chlorophyll concentration in thedosed bottles was higher than in the control bottles, indicating thatthe number of phytoplankton had been caused to increase.

As noted above, in a third aspect there is provided a method forproducing carbon credits comprising the step of sequestering carbondioxide using the method of the first or second aspect (e.g. the methodsdescribed above).

Many countries have established carbon offset markets following theguidelines of the Kyoto Protocol. Basically, industries which generatecarbon dioxide (e.g. fossil fuel burning industries) are required tooffset their carbon emissions by purchasing certified emission reductioncredits. Such emission reduction credits can, for example, be obtainedby planting trees or purchasing “clean” energy.

It is envisaged that the methods disclosed herein, which could be usedto sequester carbon dioxide from the atmosphere for a significant periodof time, could be a very cost effective way to offset an industry'scarbon emissions.

It will be appreciated by those skilled in the art that the methods setforth in the Summary are not intended to be limited by the specificembodiments described above.

In the claims which follow and in the preceding description, exceptwhere the context requires otherwise due to express language ornecessary implication, the word “comprise” or variations such as“comprises” or “comprising” is used in an inclusive sense, i.e. tospecify the presence of the stated features but not to preclude thepresence or addition of further features in various embodiments.

1. A method for removing carbon dioxide from the atmosphere, the methodcomprising delivering urea from a floating vessel to a region of aphotic zone of an ocean, whereby the number of phytoplankton is causedto increase in the region upon addition of the urea.
 2. The method ofclaim 1, wherein the region is located such that carbon dioxide issequestered from the atmosphere for a significant period of time.
 3. Themethod of claim 2, wherein carbon dioxide is sequestered from theatmosphere for greater than 100 years.
 4. The method of claim 1, whereinthe region is located at the edge of a continental shelf or over a deepocean.
 5. The method of claim 1, wherein the urea is in granular formand is mixed on the floating vessel immediately before delivery withwater taken from the region.
 6. The method of claim 1, wherein the ureais injected into the region at a predetermined depth.
 7. The method ofclaim 6, wherein the predetermined depth is between 15 and 50 meters. 8.The method of claim 1, wherein the urea is in granular form and issprinkled from the floating vessel over the surface of the ocean abovethe region.
 9. The method of claim 8, wherein the granules of ureacontain air pockets inside the granules.
 10. The method of claim 1,wherein the amount of urea delivered to the region results in theconcentration of urea in the region being between about 0.1 μmole/L toabout 10 μmole/L.
 11. The method of claim 1, wherein one or moreadditional macronutrients are delivered to the photic zone with theurea.
 12. The method of claim 11, wherein the one or more additionalmacronutrients is phosphate.
 13. The method of claim 1, wherein thefloating vessel is a boat or ship.
 14. The method of claim 1, furthercomprising the step of monitoring the increased number of phytoplankton,and adding more urea to further increase or maintain an increased numberof phytoplankton.
 15. A method for removing carbon dioxide from theatmosphere, the method comprising: delivering a source of nitrogen froma floating vessel to a region of the photic zone of an ocean, wherebythe number of phytoplankton is caused to increase in the region uponaddition of the source of nitrogen; monitoring the increased number ofphytoplankton; and adding more of the source of nitrogen to a portion ofthe region in which it is possible to further increase or maintain anincreased number of phytoplankton.
 16. The method of claim 15, whereinthe source of nitrogen is urea.
 17. The method of claim 15, wherein oneor more additional macronutrients are delivered to the photic zone withthe source of nitrogen.
 18. The method of any one of claim 15, whereinthe increased number of phytoplankton is monitored by satellite or by asecond boat or ship down current of the floating vessel.
 19. The methodof claim 15, wherein a dye is delivered to the photic zone with thesource of nitrogen to aid in monitoring the increased number ofphytoplankton.
 20. A method for producing carbon credits, the methodcomprising removing carbon dioxide from the atmosphere using the methodof claim
 1. 21. A system for producing carbon credits by removing CO₂from the atmosphere, the system comprising a floating vessel fordelivering urea to a region of a photic zone of an ocean, whereby, upondelivery of the urea to the region, the number of phytoplankton iscaused to increase.
 22. A carbon credit produced by the method ofclaim
 1. 23. A method for producing carbon credits, the methodcomprising removing carbon dioxide from the atmosphere using the methodof claim
 15. 24. A carbon credit produced by the method of claim 15.